In a matter of weeks, Southwest Colorado went from above-average temperatures and heightened wildfire danger to days of heavy rain that caused major flooding in Vallecito.
As of Friday, only 25 homes were measurably damaged by the floods, including two that had to be destroyed before they fell into the Vallecito Creek, three with major water damage and 20 with minor damage, according to preliminary findings from La Plata County. The storms come after a summer of extreme flooding events, like in the Appalachian Mountains back in September or Texas in July.
According to Jon Harvey, a geosciences professor at Fort Lewis College, fall flooding in the San Juans is not uncommon. But, he said, climate change has been found to play into storms like Hurricane Harvey, which impacted communities in Appalachia in September, and could increase the severity of storms like Priscilla and Raymond.
“Although they aren’t frequent, fall storms like this have visited the Southern Rockies long before modern anthropogenic climate change got started,” Harvey wrote in an email to The Durango Herald. “Prominent examples of higher flows on the Animas from fall storms include 1911, 1925 and 1970.”
Harvey said that in order to get flood-causing rainfall, three things must come together: an abundant moisture source, like Hurricane Priscilla and Tropical Storm Raymond; favorable winds that blow all that humid, tropical air into the Southwest; and mountains, like the San Juans, that force that moisture upward, causing it to fall as rain as the storm moves inland.
“In this case, the tropical cyclones Priscilla and Raymond helped to pump enormous amounts of water vapor into the atmosphere over the ocean near Baja Mexico, and two low pressure systems over the Western U.S. conspired to draw that tropical moisture our direction in two distinct plumes,” Harvey said.
Harvey said the amount of moisture in those storms was as much as 350-400% of the normal amount of rainfall in Southwest Colorado compared with most years. A good way to picture that amount of precipitation is to think back to the atmospheric river event in the winter of 2022-23 that buried ski resorts in several feet of snow, he said.
“So, why was there so much water vapor in the air?” Harvey asked. “Was THAT the climate change part?”
It is difficult to say, he said.
Harvey cited an article from the National Oceanic and Atmospheric Administration titled “Hurricane Helene’s extreme rainfall and catastrophic inland flooding” that found flooding killed at least 227 people and caused some $50 billion in economic losses. The article included a peer-reviewed study from the World Weather Attribution Group that found climate change was a key driver behind Helene.
“Rainfall associated with Helene was about 10 percent heavier due to climate change,” the NOAA article said.
Additionally, according to the WAGG study, weather observations indicated that severe rainfall events now occur about once every seven years in coastal regions and once every 70 years for inland regions.
“If the world continues to burn fossil fuels, causing global warming to reach 2 degrees Celsius above pre industrial levels, devastating rainfall events in (coastal and inland) regions will become another 15-25% more likely,” the study said.
But Harvey said it is too early to attribute climate change as a key factor behind the Vallecito flooding.
Isla Simpson, a scientist at the National Center for Atmospheric Research, agreed, saying there are a number of factors that play into any given weather system’s behavior. Pinning an extreme weather event on one thing could be scientifically inaccurate.
“It’s definitely hard to do a formal attribution without running some kind of experiment to really test what the impact of having warmer sea surface temperatures or warmer atmosphere is on any given event,” Simpson said.
Simpson said there is variability in what happens naturally in the atmosphere, making it difficult to say how, specifically, climate change impacts the probability, number and trajectory of storms. The atmosphere is unpredictable, and to say any of those things with certainty would require more specific experimentation, she said.
But one thing is certain, Simpson said: a warmer atmosphere can hold more moisture, which in turn can contribute to larger precipitation events.
“There are definitely some things that we can say that would happen as the atmosphere warms,” Simpson said. “The main one is that as the atmosphere warms, it can hold more moisture. And so when you have an extreme precipitation event, the chances go up that it will be more extreme.”
But, she said, for Southwest Colorado to have gone from being bone dry and in heightened wildfire danger to record rainfall and flooding in a matter of weeks is largely due to a warmer atmosphere.
“How much of this weather whiplash is caused by anthropogenic climate forcing changes I think is unclear, but I think you can expect that it would be more likely that you'd have dry periods and more extreme precipitation periods,” Simpson said.
Simpson said a warming atmosphere holds more moisture, making rainfall more severe in some areas. But in others, it evaporates moisture out of rivers, reservoirs and groundwater. That leads to prolonged periods of drought and heightened wildfire danger, she said.
“If a weather system that may have happened naturally four decades ago were to happen now, the chances are high that it would be accompanied by more precipitation,” Simpson said. “You could use the same argument for the dry periods. This warmer atmosphere is just more thirsty for water.”
sedmondson@durangoherald.com
